Kim has more longevity. I can barely pinpoint her "peak" because it feels like she just gets bigger and bigger in terms of word-of-mouth. This may also have to do with the fact that her route to fame was very... original? And that itself was enough to keep people talking about her.
Gaga's peak, while impressive, was shortlived i.e. 2008-2011. During this period, I would personally call Gaga the bigger star. But in the long run, I think Kim's cultural impact overtakes her.
Obviously Lady Gaga by a million miles, literally everyone knew (and still does know) who she was. Whereas barely any older people know who Kim is, heck, even I didn't know who she was until a few years ago.
